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Brake boosters are taken for granted in our modern world of amniotic, sealed-environment driving experiences that do everything to make driving easier, less stressful, and much safer. Nearly all modern cars, except very specialized sports cars, have a brake booster, and different types of boosters work a little differently. 

The brake booster has a simple task: help amplify the force your foot places on the brake pedal. Without a booster, the brakes would take a lot of effort and force to stop a vehicle. In short, a brake booster is essential to safety in modern vehicles.

As with any other vehicle component, they can also fail. It’s rare and usually due to age, but a brake booster malfunction can pose a serious safety risk. Let’s dig in and figure out how you can diagnose a bad brake booster.

What Is a Brake Booster?

A brake booster is usually a large vacuum reservoir or hydraulic, gas-charged piston that amplifies forces placed on the brake pedal. A vacuum or hydraulic fluid boosts brake forces and applies more brake pressure with less effort from the driver’s foot. 

Where Is the Brake Booster Located?

It is usually located in the engine bay near the brake pedal. It’s a safe bet that it will be on the driver’s side. Some cars will have it mounted low and relatively hidden; in others, the placement will be more obvious. The best way to locate it is to find the brake fluid reservoir and work back toward the driver’s seat. It will look like a large metal cylinder or a fluid pump with a sealed reservoir.

Types of Brake Boosters

There are vacuum brake boosters and hydraulic brake boosters. Most passenger cars have vacuum brake boosters that look like large cylinders; larger trucks and heavier vehicles use hydraulic brake boosters. They each fail in their own ways and require different techniques to diagnose and fix.

Symptoms of a Failing Brake Booster

Depending on the type of booster, failure can look and feel different. For a vacuum booster, the most common malfunction is loss of brake boost: When you press the brake pedal, it will either be excessively firm or you need to apply a lot more pressure to the pedal than usual. It can even feel as if the brakes don’t work. 

Since a vacuum booster operates off engine vacuum, it is an important part of an engine’s vacuum system. Sometimes a failed booster can still provide braking assist while causing a vacuum leak, causing rough engine performance and a check engine light. If you have phantom lean codes, this is a possibility.

A hydraulic booster failure will be a little more subtle. While you can outright lose braking assist, other issues might pop up. The brakes could start hanging up and not fully disengage, causing excessive wear or heat issues, or a dashboard warning light might illuminate. On some cars, the hydraulic booster is tied to the anti-lock brakes, so the car can sense a failure. Either way, failures from a booster will affect all wheels or the brake pedal, never an individual wheel.

How To Test Your Brake Booster

First things first, do some basic checks. For a vacuum booster, make sure the vacuum line that feeds vacuum to the booster is in good condition and isn’t cracked. Next, start the car for a moment and shut it off. Hop in and press the brake pedal and release. Do it two more times. If you can feel the assist for two pumps after shutting the car off, it’s good. It should harden on the third pump. A bad booster will leak vacuum and lose assist more quickly or not have any at all.

For a hydraulic booster, inspect the booster for any fluid leaks. Some hydraulic boosters use the power-steering system, and some work independently. If applicable, inspect power-steering lines for leaks and check the power-steering fluid level. Perform the same test as the vacuum booster to verify brake assist. If there is a warning light, plug in a scan tool and check for trouble codes.

How Do I Know It’s Not a Problem With the Master Cylinder?

Master cylinder problems usually manifest themselves as brake fluid leaks and have the opposite effect of a failing booster. Instead of the pedal hardening, it will soften and not hold pressure until it hits the floorboard. It is much more dangerous to have a failed master cylinder than a brake booster. A leaking master cylinder can sometimes cause the booster to fail.

FAQs About Brake Boosters

Car Bibles answers all your burning questions. 

Q. How much does a brake booster cost to replace?

A. It can cost anywhere from $50 to $400 or more for parts, depending on the type of booster and type of car. Labor can range from $150 to $250 for a total cost of $200 to $650 on average.

Q. Can you drive with a faulty brake booster?

A. You can, but it is not recommended. It will take excessive force to stop your car, which could make an emergency stop impossible. Get it fixed immediately.

Q. Will brakes still work with a faulty brake booster?

A. In most cases, yes. It is designed to get you home in the worst-case scenario. Drive with caution or get towed, if possible.

Q. Why do I hear air when I press on my brakes?

A. That is the vacuum brake booster using vacuum to assist with braking.

Q. Can a brake booster engage the brakes on its own?

A. Some can, and they are hydraulic brake boosters. Beware of any brake smells or excessive wear on all four wheels.

Learn More From This Video Tutorial About Brake Boosters

Some of us don’t learn that effectively with words, myself included, so here’s a video by the folks at Rock Auto. It demonstrates how brake boosters work and how to diagnose a brake booster failure.

Disclosure: Carbibles.com is also a participant in the Amazon Services LLC Associate Programs, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. Pages on this site may include affiliate links to Amazon and its affiliate sites on which the owner of this website will make a referral commission.

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